Frequency modulation signaling system



l atented Jnly 6,

UNITED STATES PATENT CFFICE FREQUENCY MODULATION SIGNALING SYSTEM Edwin H. Armstrong, New York, N. Y.

Application October 12, 1940, Serlal No. 360,882

Claims. (Cl. 2506) This invention relates to improvements in the phase shifting method of generating frequency modulation. Its object is principally the simplification of the apparatus, particularly in those cases where an extensive frequency range must be transmitted.

In the phase shifting method of generating frequency modulated waves the process, as is well known, consists in varying the phase of a current derived from a fixed frequency sourc by an amount directly proportional to the amplitude of the modulating current and inversely according to its frequency and multiplying the resultant phase shift by a series of frequency multiplications to the degree necessary to produce the desired frequency change. While special methods have been devised to produce greater initial phase changes, in the simple form the phase shift is limited to about 30 plus and minus. This is the maximum allowable at the lowest modulating frequency which can be employed without exceeding allowable distortion limits for high quality transmission. In present day practice it is customary to'transmit a frequency range of 30 to 15,000 cycles and to produce, at full modulation a frequency deviation of plus and minus 75 k, c. This involves a frequency multiplication of several thousand fold (three or more thousand) in order to obtain the desired frequency change at the lowest modulating frequency.

Present day frequency modulation broadcasting operates in the 40 megacycle band. Suppose a frequency multiplication of four thousand is required. This would mean that the initial frequency would vbe ten thousand cycles. It is not possible to modulate a current having a frequency of ten thousand cycles with one of fifteen thousand cycles so that the multiplication must be obtained in two stages. This is accomplished by starting at some frequency which can be readily modulated, as, for example, 100,000 or 200,000 cycles, multiplying it by a series of doublers or triplers to some high value (of the order of 12 megacycles) heterodyning it down to some submultiple of the frequency to be transmitted and remultiplying it again up to the transmitting frequency. In this way it is possible to obtain all the frequency multiplication desired.

There are numerous types of services such as portable relay equipment in broadcast pickup work where the amount of quipment necessary to carry out the process just described is undesirable yet where a highly stable type of transmission, such as the phase shift method, is required.

In some instances the frequency multiplication ob- 55 whi h ont ins a load resistance Ill across which tainable by the direct method falls short of the desired amount by a relatively small value. Such a case occurs in the relay broadcast band now in use in the vicinity of 150 megacycles. For this frequency of transmission, if the initial crystal or oscillator frequency is 150 k. c., a direct frequency multiplication of 1000 fold can be obtained and on the basis of, for example, a 4000 fold required multiplication, this falls only 4 times short of the required amount. Stated in another way, the desired frequency multiplication can be obtained for all frequencies of modulation except those in that range running from the lowest frequency to four times that value. In practical broadcasting this means from 30 cycles to cycles, the 30 phase shift maximum allowable being reached at 120 cycles.

It is the purpose of this invention to provide means which may be employed which permit the use of the direct multiplication method while compensating for the deficiency and distortion at the lower end of the modulation frequency range so that a. uniform frequency characteristic may be obtained without increase in harmonic distortion. This is accomplished by modifying the correction network employed in the input modulation circuit in such manner that while it functions in the usual way above 120 cycles to produce a phase shift which is inversely proportional to the modulation frequency, at or below this frequency the phase shift tends to become constant and to be limited to 30 maximum. This result is produced'and the effect on the resulting frequency characteristic in the receiver is compensated for in the manner hereinafter described.

Referring now to the drawings which form a part of this specification, Fig. 1 shows the general arrangement of the transmitting system and Fig. 2 illustrates the frequency characteristic thereof. Fig. 3 shows the general arrangement of the receiving system and Fig. 4 illustrates the frequency characteristic thereof. Fig. 5 shows the resultant characteristic and the individual characteristics of the transmitting and receiving systems.

Referring specifically to Fig. 1, i represents an oscillator, preferably crystal controlled, 2 the usual carrier amplifier with the balanced modulator 3 arranged in the conventional manner. 4, 5, 6 represent frequency multipliers and I the power amplifier for the frequency to be radiated. 8 represents the input circuit for the signal modulating current controlling a vacuum tube I will be referred to hereinafter.

is connected a correction network ll, 12 which amplifier whose grid circuit is connected across the capacity I2 of the correction network in the usual way. The output of I3 controls the balanced modulator 3 through the transformer M. The amplifying system is similar in principle to the circuit shown in my Patent #1,941,068 except for certain differences in the proportioning of the constants which will appear hereinafter. In the usual arrangement, the resistance II is made so high with respect to the impedance of the capacity l2 for the lowest frequency of modulation that the current through the circuit remains constant regardless of frequency. Consequently the voltage across the capacity l2 varies inversely as the frequency and the phase shift created by theaction of the balanced modulator likewise varies inversely as the frequency. This results in a frequency deviation in the transmitted wave which is independent of the modulating current frequency. In accordance with the present invention the resistance II is made less than necessary to maintain the current constant over the entire modulating frequency range so that at the lower end thereof the current is limited by the impedance of the capacity I2. As a consequence of this the phase shift tends to become constant for the lower frequencies and the frequency deviation falls off as indicated in Fig. 2. The shape of this curve is determined by the relative proportions of the resistance II and capacity l2. In the above considerations the impedance of II must be high compared to the value of the resistance l0.

Thefrequency characteristic of the received signal will of course be a replica of the curve of Fig. 2 unless means are taken to correct for it. In accordance with the invention, such means are provided as shown in Fig. 3. In this figure l5 represents the usual converter and it the usual oscillator of a superheterodyne receiving system. H, l8 represent the intermediate frequency amplifiers, iii the limiter, 20 the discriminator and 2| the detection system of the usual frequency modulated receiver. In the detection output circuit a network is provided which is the complement of the network ll, I2 at the transmitter. In the receiver network the resistance 22 is made sufiiciently high to maintain the current through the circuit constant for all frequencies of modulation. 23 is a resistance which is small compared to the resistance of 22 but large compared to the reactance of the capacity 24 for the frequencies at which the deviation of the transmitter is a constant quantity. By properly proportioning the two impedances the resulting frequency characteristic of the network 22, 23, 24 may be given the desired shape to complement the characteristic of the network ll, [2, Fig. 5 illustrates this result.

It will be understood that the use of a network at the receiver to raise the low frequency currents in amplitude with respect to the remainder of the range will likewise increase the noise components at those frequencies. These noise components are of two sorts. One is that due to static, tube noise, etc., originating generally from radio frequency currents and the other that due to the residual hum in the receiver and having its origin principally in the audio system, or in the cathode heater current supply. In regard to the first named type of noise, it is not of much importance as due to the characteristic of frequency modulation reception the distribution of the noise voltl3 represents an tage after detection is proportional to frequency and in the ranges which are being augmented it is extremely low. With regard to the second type. of noise, namely, that originating in the power supply, the augmenting of the low frequency range is important and may result in increase in the hum from 60 cycle and cycle components. In relay receivers, unlike broadcast receivers, it is practical to take the necessary precautions, such as providing D. C. heater supply, and extra care in filtering to remove disturbances from these sources.

It will be understood, of course, that in addition to the networks illustrated, it is practicable and useful to employ the usual predistorting and restoring networks adapted for speech and musical transmissions.

I have described what I believe to be the best embodiment of my invention. I do not wish, however, to be confined to the embodiment shown. but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. The method of producing a frequency modulated current from a source of fixed phase and frequency, which consists in varying the phase of a current derived from the source by an amount which is directly proportional to the amplitude of the modulating current and inversely proportional to the frequency of said modulating current throughout a range of modulating frequencies while varying the phase of said current by a substantially constant amount by modulating frequencies lower than the lowest frequency of said range, and multiplying the resultant phase shift a sufficient number of times to produce the required degree of modulation.

2. The method of producing a frequency modulated current from a source of fixed phase and frequency, which consists in varying the phase of a current derived from the source by an amount which is directly proportional to the amplitude of the modulating current while also varying the phase of said current by an amount which is inversely proportional to the frequency of the modulating current throughout the upper portion of the range of frequencies to be transmitted and by a substantially constant amount throughout the remaining portion of the range of frequencies to be transmitted and multiplying the resultant phase shift a sufiicient number of times to produce the required degree of modulation.

3. A transmitter for frequency modulated waves comprising, in combination, a source of current of fixed frequency and phase, a modulating device connected to said current source and having an input circuit, a correction network coupled to the input circuit of the modulating device and adapted to receive the modulating current, said correction network comprising means for causing the modulating device to vary the phase of the current derived from said source by an amount which is directly proportional to the amplitude of the modulating current while also varying the phase of said current by an amount which is inversely proportional to the frequency of the modulating current throughout the upper portion of the range of frequencies to be transmitted and by a substantially constant amount throughout the lower portion of the range of frequencies to be transmitted and means for multiplying the resultant phase shift a sufilcient number of times to produce the required degree of modulation 4. The transmitter as set forth in claim 3 in which the correction network comprises a vacuum tube, a resistor through which the modulating current flows, and a circuit shunted across said resistor and comprising the series connection of a second resistor and a condenser, the common terminal of the second resistor and condenser being connected to the grid of said vacuum-tube.

5. The method of signaling which consists in varying the phase of a current by an amount which is directly proportional to the amplitude of the modulating signal current while also varying the phase of said current by an amount which is inversely proportional to the frequency of the modulating current throughout the major portion of the frequency range to be transmitted and by a substantially constant amount throughout the lower portion of the said frequency range, multiplying the resultant phase shift a sufficient number of times to produce the required degree of modulation, transmitting the resulting frequency modulated currents, receiving the transmitted currents, detecting the frequency modulation changes, and increasing the amplitude of the detected currents throughout the lowest portion of the range of modulating current frequencies with reference to the amplitude thereof throughout the remainder of said range.

6. A frequency modulation signaling system, comprising at the transmitter, a source of current of fixed frequency and phase, a phase shifting device connected to said source and havin an input circuit, a correction network coupled to the input circuit of the phase shifting device and adapted to receive a modulating current, said correction network comprising means for causing the phase shifting device to vary the phase of the current derived from said source by an amount which is directly proportional to the amplitude of the modulating current while also varying the phase of said current by an amount which is inversely proportional to the frequency of the modulating current at the upper portion of the range of frequencies to be transmitted and by a substantially constant amount in the lower portion of the range of frequencies to be transmitted, and means for multiplying the resultant phase shift a suflicient number of times to produce the required degree of modulation, said system comprising at the receiver, means for receiving the waves, means for detecting the frequency modulated signals, and a frequency compensating network connected to the output of said detecting means, said network comprising impedance devices for increasing the amplitude of the detected currents throughout the lower portion of the range of modulating frequencies with reference to the amplitude thereof at the upper portion of said range.

7. A receiver for receiving frequency modulated signal waves in which the frequency deviation of the transmitted wave progressively increases from the lowest frequency of the band of signal frequencies to be transmitted to an intermediate frequency of the said band. said receiver comprising, in combination, means for receiving the waves, means for detecting the frequency-modulated signals, and a frequency compensating network connected to the output of said detecting means for increasing the amplitude of the detected currents throughout the lower portion of the range of modulated frequencies with reference to the amplitude thereof at the intermediate portion of said range, said network comprising the series connection of at least one resistor and a condenser, the resistance of the resistor being substantially greater than the impedance of the condenser for frequencies within the intermediate and upper portion of the range of modulating frequencies.

8. The method of reproducing the signal modulations of a frequency modulated radio wave the frequency deviations of which progressively increase from the lowest frequency of the band of signal modulations transmitted by said wave to an intermediate frequency of the said band which comprises, receiving the wave, detecting the frequency modulation changes and increasing the amplitude of the detected currents throughout the lower portion of the range of modulating current frequencies with reference to the amplitude thereof throughout the middle portion of said range.

9. The method of reproducing the band of audio signal modulations of a frequency modulated radio wave the frequency deviations of which progressively decrease from a signal frequency of substantially cycles to a signal frequency of substantially 30 cycles which comprises, receiving the wave, detecting the frequency modulation changes and progressively increasing the amplitude of the detected currents as the frequency thereof decreases from substantially 120 to 30 cycles.

10. A receiver for reproducing the band of audio signal modulations of a frequency modulated radio wave, the frequency deviations of which progressively decrease from a signal frequency of substantially 120 cycles to a signal frequency of substantially 30 cycles, said receiver comprising, in combination, means for receiving the waves, means for detecting the frequency modulation changes and a frequency compensating network connected to the output of said de tecting means, said network being arranged progressively to increase the amplitude of the de-' 'tected currents as the frequency thereof decreases from substantially 120 to 30 cycles.

EDWIN H. ARMSTRONG: 

